First week of development

1. First week of development Fertilization

3. FERTILIZATION • fusion of two mature germ cells an ovum and a spermatozoon, to form a mono nucleated single cell- zygote. • The fusion occurs in the ampulla of the uterine tube. • Fusion of 2 highly specialized cells leads to formation of an unspecialized, undifferentiated, diploid zygote.

4. Mechanism • Approximation of the gametes • Contact and fusion of the gametes • Effect of fertilization

5. APPROXIMATION OF GAMETES • transport of the sperms and ova in the female genital tract to reach the uterine tube. • How does the sperm get to uterine tube? • Chemotaxis between gametes (somewhat) • Mechanical factor (main stuff)

6. Mechanical factor • Semen deposited during coitus in the vagina • Prostagladins from the semen trigger power uterine contraction • mixture of sperms and uterine secretion. • Spermatozoa reduced by -dilution of mucous secretion -barrier of the constriction of cervix and the uterine ostium Natural selection method for viable sperm

7. How does ovum get to uterine tube • ciliary beats of the uterine tube • rhythmical contraction of uterine tube musculature.

11. Contact and fusion • CONTACT AND FUSION OF GAMETES • A direct contact between spermatozoa and zona pellucida of ovum is necessary before actual fusion of gametes. • But the cells of corona radiata provides obstacle to the penetration of the sperm • It must be subdued!

13. Challenge of penetration • corona radiata - granulosa cells • zona pellucida - polysaccharide shell

14. FUSION OF THE SPERM NUCLEI AND OOCYTE COMPLETE FERTILIZATION. • membrane changes to prevent multiple sperm from fertilizing ova • a. Sperm penetrates corona radiata • b. Attaches to zona pellucida • c. Acrosome of sperm releases trypsin like enzyme • d. Sperm penetrates • e. Egg membrane depolarizes ( Na diffuses in, Ca out) • f. Cortical granules released • g. Zona hardens • h. Prevents multiple fertilizations • i. Secondary Oocyte divides • j. Polar body formed, egg and sperm nucleus fuse

15. Pre-fertilization processing of sperm • the sperms undergo 2 processes: • physiological change – capacitation • structural change – Acromosomal reaction.

17. Capacitation • period of conditioning in the female genital tract which last for 7 hours. • involves the shedding of the plasma membrane (glycoprotein coat and plasma protein of the head at the acrosome) • It permits acrosomal reaction to take place.

18. Acrosome reaction • occurs close to the vicinity of Oocyte • influenced by substances released from the corona radiata and the oocyte. • induced by angiotensin converting enzyme in the acrosome. • involves the perforation of the acrosomal cap of the head of sperm, to allow the passage of enzyme that will digest a path for the sperm through the corona radiata and zona pellucida

19. Substances released during acrosomal reaction • hyaluronidase (for penetration of corona radiata barrier) • Acrosomal enzymes digest zona pellucida • They are esterase,acrosin, neuraminidase • They cause lysis of zona pellucida • (b) trypsin – like subs (for digestion of zona pellucida) • (c) Acrosin (for penetrating zona pellucida also) • Tubal mucosal enzyme also assist hyaluronidase passed corona radiata.

20. Note • The fertilizin – antifertilizin reaction is specific and essential feature for the attachment of gametes. • The fertilizing sperms penetrate the zona pellucida by lytic action of hyaluronidase

21. note • attachment between zona pellucida and the plasma membrane of the sperm is a reaction between antisera and antibody. • The zona pellucida elaborates FERTILIZIN (glycoprotein) on the surface of spermatozoa for Agglutination of sperm. • sperm secretes ANTIFERTILIZIN which agglutinates the ova.

22. notes • About 200 – 300 million spermatozoa released, only about 300 to 500 reach the fertilization site • Others help fertilizing spermatozoa penetrate the barrier of the corona radiata • only one sperm cell penetrate the zona pellucida to fertilize the ovum. • The zona pellucida reacts and prevents the entry of more sperm cells-zonal reaction. • This zonal hardening reaction is due to certain cytoplasmic substances produced by the ovum (fertilizin).

23. Result of fertilization • (a) Completion of 2nd meiotic division of the female gamete – the secondary Oocyte completes the 2nd meiotic division with the extrusion of 2nd polar body in the periviteline space. The nucleus became vasicluar and forms female pronucleus, containing haploid chromosome (22 + x). • (b) Restoration of diploid number of chromosome in the zygote – the head of the spermatozoa containing 23 chromosome swells and form male pronuclei. The male and female pronuclei meet each other near the centre of the ovum and nuclear membrane disappears. This results in nuclear fusion with restoration of diploid number of chromosomes. • (c) Determination of chromosomal sex • (d) Initiation of cleavage division of zygote • (e) Determination of the polarity of embryo – caphalo – caudal polarity of embryo

24. Sex determination

25. based upon whether an X or Y carrying sperm has fertilized the egg should be 1.0 sex ratio is actually 1.05, 105 males for every 100 females. some studies show more males 2+ days after ovulation cell totipotential

26. Men- Y Chromosome • carries sry gene • gene activates pathway for male gonad

27. Women- X Chromosome • one X chromosome in each cell has to be inactivated. • process is apparently random • therefore 50% of cells have father's X, 50% have mother's X

28. Cleavage of the Zygote • Series of rapid mitotic division • 30hrs after fertilization cleavage of zygote forms 2 blastomeres • cleavage with no cytoplasm synthesis • therefore individual cell volume decreases • initially synchronously, the asynchronously • slow- centre cells, larger • fast- peripheral cells, flattened against zona pellucida

30. First week of Development

31. zygote • the ultimate stem cell. • It is totipotent (it can produce all types of human body cells for the embyro), • and it produces the life support system (extra-embryonic membranes • Within 24-48 hours after fertilization, early pregnancy factor (EPF) can be detected in the maternal serum.

33. 1. Zona pellucida2. Blastmere

34. Morula • solid ball of 12-16 cells, formed following rapid increase in the number of the cells. • 3 after the fertilization, • 4 days after fertilization, morula enters the uterus, • fluid-filed spaces appear between the blastomeres and fuse into a central cavity called the blastocoele.

37. Early cell division • Formation of the Morula • there are now 16-20 cells • at the end of the oviduct • ball of cells • Mulberry-like.

38. Formation of the Blastocyst • Day 5 • zona pellucida lost by zonal- hatching • broken down by uterine secretions • compaction • central cells now compacted • located on a region of the wall

39. compaction • After 9-cell stage, blastomere change shape,tightly align with each other. • Form a compact ball of cell- compaction process. • Mediated by cell surface adhesion glycoproteins • Permits greater cell to cell interaction • Compaction is important for segregation of inner cell mass.

41. Molecular basis • Blastocyst formation begins at day 5 after conception in humans • Differential gene expression in the morula is thought to be the cause of the lineage divergence of different cell types. • the Oct-3/4 transcription factor is restricted to the Inner Cell Mass, • Cdx2 is expessed at a higher level in the trophoblast than the ICM. • This differential transcription factor expression is likely to be the result of positional effect – • cells in the middle of the preceding zygote are in a different environment to those on the outside, • thus causing differential expression.

42. Blastocyst • blastoceol formation • cavity formed by compaction • fluid filled blastoceol

44. Blastocyst • Embryoblast • Inner cell mass • forms entire embryo • Trophoblast • forms the placenta • placental membranes